This invention concerns improvements in or relating to lenses and relates more particularly to transfer lenses.
A transfer lens functions to transfer visual information from a position or positions at which the visual information is presented on a primary display means to another position at which it is viewable by an observer or at which further optics, such as a field lens, act. Usually the transfer lens effects a relatively low magnification. The primary display means may comprise two primary displays from which light is combined so that the transfer lens produces a combined image of the two displays.
A particular example of use of a transfer lens is in an aircraft head-down display which enables the pilot to view simultaneously visual information primarily displayed on a cathode ray tube and on a map screen. In this example the exit pupil of the transfer lens is re-imaged by a field lens at a position in the vicinity of the pilot's head. In order to permit the pilot considerable freedom of head movement whilst still being able to see the combined image of the cathode ray tube display and the map display a large diameter final exit pupil is required. A high aperture transfer lens is therefore needed. Further, there is generally a requirement that the physical space occupied by the optics, and in particular the distance between the cathode ray tube/map screen and the field lens, should be kept to a minimum. The transfer lens therefore needs a wide angular field of view.